legionum

Legionum is the genitive plural form of legio, a Latin noun meaning “a legion,” the principal unit of the Roman army. Legio is a feminine noun of the third declension; its nominative plural is legiones, and its genitive plural is legionum. The form legionum appears in Latin texts to express “of the legions” or “the legions’,” typically indicating possession, origin, or association, as in phrases such as dux legionum (leader of the legions) or impedimenta legionum (the equipment of the legions).

A Roman legion was a major military formation, usually comprising about 4,000–6,000 infantry with cavalry support, organized into cohorts and centuries, and commanded by a legatus or other senior officer depending on period. Legions were central to Roman campaigns from the late Republic through the imperial era, and the word legionum occurs frequently in historical, military, and administrative writings to denote multiple legions or the possessions of the legions.

In usage, legiones refers to the units themselves (the legions), while legionum denotes possession or association with the legions. Modern Latin scholarship treats legionum as the standard genitive plural form for legio in textual contexts involving multiple legions or their attributes. See also legio, legiones, Roman army, legatus.